You are on the right track in understanding the purpose of your son's behavior. His behavior is a form of self soothing. He will eventually stop this, so you needn't be alarmed. I say this because you point out that he is developing well and appears to be happy. Sometimes the best thing we can do is exercise our patience.
My son was also a sleeve sucker, he is now 9 and has been shirt sleeve free for about a year now. With high stress it will come back for a short period of time 2-3 hours in a bad day.It really will end if you remind him to " get that thing out of your mouth" Good Luck